Nutrient losses in runoff from feedlot surfaces as affected by unconsolidated surface materials

Beef cattle feedlots contain unconsolidated surface materials (loose manure pack) that accumulate during a feeding cycle. The effects of varying amounts of unconsolidated surface materials on runoff nutrient losses are not well understood. The objectives of this study were to (1) compare runoff nutrient losses from feedlot surfaces containing varying amounts of unconsolidated surface materials, (2) determine if differences in runoff nutrient losses exist among rainfall simulation runs, (3) relate runoff nutrient losses to selected feedlot soil characteristics, and (4) identify the effects of varying runoff rate on nutrient loss rates from feedlot surfaces. This study was conducted on 0.75 m wide by 2 m long (2.47 ft wide by 6.58 ft long) plots containing 0, 6.7, 13.5, or 26.9 kg m (0, 1.37, 2.77, or 5.51 lb ft−2) of unconsolidated surface materials. Simulated rainfall was applied during three 30-minute events that were separated by 24-hour intervals. Inflow was added at the top of all plots during selected tests to examine the effects of varying flow rate on nutrient loss rates. No significant differences in the measured water quality parameters were found among the surfaces containing varying amounts of unconsolidated surface materials. Measurements of dissolved phosphorus, particulate phosphorus, total phosphorus, ammonium nitrogen, chloride, total dissolved solids, electrical conductivity, and erosion consistently decreased during the three rainfall simulation runs. Runoff losses of ammonium nitrogen (NH4-N), total nitrogen, and nitrate nitrogen were all correlated to easily obtained soil EC measurements. All measured water quality parameters were significantly influenced by runoff rate. Thus, runoff rate, and not the amount of unconsolidated surface materials on the feedlot surface, significantly influenced nutrient losses in runoff.